This invention relates to a magnetic disk drive and error correction method.
The constitution of the conventional magnetic disk drive is shown in FIG. 1.
The magnetic disk drive is a device for recording information onto a magnetic recording medium 100 by use of a read/write head 200 (hereinafter referred to as "head") and for reproducing the information recorded on the magnetic recording medium by use of the head 200.
As shown in FIG. 1, the circuit for controlling the magnetic disk drive comprises eight blocks: Central Processing Unit (CPU) 1; Read Only Memory (ROM) 2; Random Access Memory (RAM) 3, Hard Disk Controller (HDC) 4; Read/Write (R/W) circuit 5; servo-control circuit 6; spindle motor control circuit 7; and head amplifier 8.
The operations of the above-mentioned blocks will be described below.
CPU 1 controls all the parts of the magnetic disk drive. The control process is executed in order of the steps recorded on the ROM 2.
ROM 2 stores the control steps of the magnetic disk drive.
RAM 3 temporarily stores the data transmitted to the HDC 4 from a host computer (not shown) and the R/W circuit 5, and functions as a memory for the CPU which operates on the basis of the sequences recorded in the ROM 2 in advance.
The HDC 4 interfaces between the host computer and the magnetic disk drive.
The R/W circuit 5 encodes the data transmitted from the HDC 4 in the form suitable for the magnetic recording, and decodes the data transmitted from the head amplifier 8.
The servo-control circuit 6 positions the head 200 on a designated cylinder in accordance with the instruction from the CPU 1.
The spindle motor control circuit 7 controls the rotation speed of the magnetic recording medium in accordance with instructions from the CPU 1.
The head amplifier 8 amplifies the signal read out by the head 200 and controls the electric current flowing into the head 200 in accordance with write data transmitted from the R/W circuit 5.
The basic operation of the magnetic disk drive having the above-mentioned elements will be described below.
The data recorded on the magnetic recording medium is read by the head 200, and amplified by the head amplifier 8. The data amplified by the head amplifier 8 is transmitted to the R/W circuit 5. The data transmitted to R/W circuit 5 is demodulated into the original form and transmitted to the HDC 4.
FIG. 2 is a block diagram showing the schematic constitution of the R/W circuit 5. The R/W circuit 5 has AGC circuit 51; analog filter 52; A/D converter 53; digital filter 54; Viterbi decoder 55; and demodulator 56. The AGC circuit 51 adjusts (amplifies) the amplitude of the output signal output from the head 200 such that a constant level of amplitude can be obtained irrelevant of the magnitude of the output signal. The analog filter 52 transmits only signals with frequencies within a predetermined band. The A/D converter 53 converts the input analog signal passing through the analog filter 52 into a digital signal. The digital filter 54 transmits the digitalized signal with frequencies within a predetermined band. The Viterbi decoder 55 decodes the digital signal passing through the digital filter 54. The demodulator 56 demodulates the decoded data into the original form.
In the general magnetic disk drive, the HDC 4 adds redundant data of several bytes to the data transmitted from the host computer when the transmitted data is written in the magnetic recording medium, in order to correct reading errors.
By use of the transmitted data and redundant data, the HDC 4 checks whether or not the data transmitted from the R/W circuit 5 is correct when the data is read, and corrects the data which can be corrected, in real time (this correction method is called "ON-THE-FLY correction" and is widely used).
The number of the errors which can be corrected by "ON-THE-FLY correction" increases as the redundant data length is lengthened, but the formatting efficiency decreases, in contrast. Therefore, the operating system is designed to have the optimum redundant data length which is determined in view of both the reading error rate and the formatting efficiency.
The upper limit of the number of correctable data errors which can be corrected with a predetermined redundant data length depends on the correction-error rate. Generally, when the upper limit of correctable data errors is raised without lengthening a predetermined redundant data length, the correction-error rate increases. Accordingly, in the conventional magnetic disk drive, the maximum values of the redundant data length and the number of correctable data errors are determined in view of the formatting efficiency, the reading error rate before the error correction, and the correction-error rate.
In recent days, a MR head is used for reading data in a magnetic recording medium. However, when the MR head is used, friction heat (called "thermal asperity") generated between the magnetic recording portion of the head and the protrusions of the magnetic recording medium causes distortion in the signal waveform, as shown in FIG. 3.
If the distortion is generated, the data in the distortion portion cannot be correctly read, and reading errors occur. The errors can be corrected by the HDC 4 when the error length is ranged within the scope of the length which can be corrected by the ON-THE-FLY correction.
However, the length of the error which can be corrected by the ON-THE-FLY correction is calculated in view of the correction-error rate, and fixed at a value, as mentioned before. Accordingly, if the length of errors which are caused by the thermal asperity as shown in FIG. 3 even slightly exceed over the correctable error length, the HDC 4 cannot correct the errors.
As described above, the conventional magnetic disk drive has a problem that error-correction cannot be executed when the length of the error which is caused by the thermal asperity exceeds the correctable error length.